This is the third BYGL Alert! this season that focuses on Fall Webworm (Hyphantria cunea). This Alert is in response to the numerous e-mail reports I've received of spectacularly large silk nests occurring in southwest Ohio. They are most likely the work of the red-headed fall webworm biotype.
You can read the past postings including the Alert on the rise of second-generation caterpillars by clicking on these hotlinks:
Our native fall webworms have two biotypes named for the color of their head capsules. The black-headed biotype has black head capsules. Nest produced by the caterpillars of this biotype appear to include caterpillars from only a few egg masses. The nests tend to be small and compact usually enveloping only a dozen or so leaves. However, several of these small communal nests may be found on the same branch.
Red-headed fall caterpillars are far more cooperative; their communal nests may include caterpillars from a large number of egg masses. Thus, they can produce some truly spectacular multilayered nests enveloping the leaves on entire branches. This biotype is the more damaging of the two.
Historically, red-headed fall webworms were confined to the northeast and eastern parts of Ohio and black-headed webworms were found elsewhere in the state. However, in 2016, I found the red-headed biotype in a Hamilton County park near the Ohio-Indiana border. Since then, this biotype has expanded its local geographical range to include several counties in the southwest part of the state.
Fall webworms have a very wide plant host range with their silk nests recorded on over 400 species of trees and shrubs. However, penetrating the webworm's dense silk nests with topical insecticides, particularly second-generation nests, is problematic.
The insecticide option is seldom justified for managing fall webworms. First, there are over 50 species of parasitoids and 36 species of predators known to make a living on fall webworms. They are the primary reason year-to-year fall webworm populations can rise and fall dramatically. Indeed, insecticides may kill these bio-allies preventing them from positively influencing the population dynamics of their webworm prey.
Second, fall webworms seldom cause significant injury to the overall health of established host trees. Most of the damage is done by the second generation. Their late-season defoliation occurs after trees have acquired enough carbohydrate to support next season's leaf expansion. This includes the potential damage caused by the large nests produced by red-headed webworms.
2-Step Digital Management Demonstration
Given the limited impact of fall webworm on overall tree health coupled with the high impact of bio-allies, a perfectly valid management option for established trees is to do nothing. Recently planted trees are a different matter. Thankfully, silk nests on small trees are usually within easy reach.
Julie Molleran (Horticulturist, Spring Grove Cemetery and Arboretum) demonstrates the 2-Step Fall Webworm Digital Management Technique. This approach is in keeping with Spring Grove's dedication to using all facets of Integrated Pest Management (IPM).
Julie is taking advantage of several fall webworm behavioral characteristics to maximize impact. First, both red-headed and black-headed fall webworms only eat leaves that are enveloped by their silk; they never leave their nests. In fact, the caterpillars are well-equipped for life on the web. Their long hairs aid the caterpillars in remaining positioned within the webbing with their hairs folding backward making them look like they're "swimming" through their webs.
Second, webworm caterpillars are often found grouped together in dense clusters making it handy to remove the whole colony with a single swipe. The entire silk nest does not need to be removed other than for aesthetic reasons.
Using pruners to cut out nests is not needed unless nest removal fits with overall pruning plans. Of course, setting fire to the silk nests is highly discouraged unless there is a desire to deal with the severe bark injury produced by 1,000+ F. temperatures.
As Julie demonstrates, physically removing webworm caterpillars coupled with the "caterpillar stomp" is a highly effective management option. Thus far, no populations have become resistant to this IPM tactic.